This invention relates to a magnetic fluid sealing device for sealing relatively movable surfaces of rotating shafts, sleeves and the like to maintain a pressure difference across seals or to prevent leakage of a lubricant along the surfaces.
In relatively moving surfaces which may be lubricated, it is sometimes necessary to interpose seals therebetween to prevent leakage of a lubricant or gas or to maintain a pressure difference across the seals where one of the surfaces such as a rotating shaft passes from one environment at a first pressure into a second environment at another pressure.
As such seals, it has been suggested that magnetic fluid be employed in the gap between relatively movable surfaces. The magnetic fluid comprises a carrier fluid such as water, a hydrocarbon, a fluorocarbon, or a fatty acid and magnet-type particles such as ferrite mixed in the carrier, and is captured in the gap by magnetic flux generated by one or more permanent magnets. In such a magnetic fluid seal, as the relatively movable surfaces do not directly contact each other, they are subject to hardly any wear, whereby the serviceable life of the seal is remarkably extended in comparison with those of mechanical seals. In addition, it provides a positive seal. Therefore, the magnetic fluid seal is often applied to an apparatus used in the environment of a high vacuum such as an X-ray tube apparatus.
Magnetic fluid sealing devices of this kind are disclosed in British Pat. No. 783,881 and U.S. Pat. No. 3,620,584. In FIG. 5 of the British patent, a shaft of magnetic material is rotatably accommodated in a housing. A plurality of annular permanent magnets are fixed to the inner surface of the housing in series in the longitudinal direction of the housing with annular polepieces, each being held between two adjoining permanent magnets. Between each of the inner peripheries of the polepieces and the outer surface of the shaft are formed sealing gaps in which the magnetic fluid is entrapped and retained. The two adjoining permanent magnets on opposite sides of each of the polepieces are so arranged that their polarities are symmetrical with respect to that polepiece, that is, the permanent magnets are arranged in the sequence of "N.S-polepiece-S.N-polepiece-N.S". This arrangement of the permanent magnets is the same as that of the present invention as described hereinafter. In addition, each of the inner peripheries of the polepieces is bevelled so as to form an annular upheaved portion in its central portion in the axial direction of the polepieces thereby to provide the desired concentration of the magnetic field. Magnetic flux generated from one of the magnets passes through one of polepieces across one of the gaps and through the shaft and back through another gap and polepiece to the magnet to complete a magnetic flux circuit.
The two magnetic flux flows generated from two adjoining permanent magnets in each polepiece pass across each gap through the upheaved portion of the polepiece in the same direction, and accordingly the two magnetic flux flows repulse each other to diverge and provide a relatively wide magnetic field for holding the magnetic fluid in the gap. Furthermore, to avoid magnetic saturation of the polepieces, the thickness of each polepiece is limited, and a very thin plate cannot be used as a polepiece.
Therefore, as the magnetic flux density in the gaps between the inner peripheries of the polepieces and the circumferential surface of the rotating shaft is not very high, the holding force, for holding the magnetic fluid, generated by the magnetic flux passing through each gap is not very great. To increase the holding force, it is necessary to use one or more magnets with strong magnetic field or to form narrow gaps, for example, of less than 20.mu.. Accordingly, it is difficult to assemble the sealing device so as to maintain narrow gaps between the rotating shaft and the polepieces. In addition, if the number of permanent magnets is increased to increase the number of barriers to confine the magnetic fluid, the sealing device will become large and bulky.
In the U.S. Pat. No. 3,620,584, a magnetic fluid sealing unit is positioned between two ball bearings as shown in FIG. 5 of its accompanying drawings. The fluid sealing unit comprises an annular permanent magnet and two polepieces on opposite sides of the magnet. The inner periphery of each polepiece is concave in the shape of a triangule in radial section of the annular polepiece so as to form two knife edges on opposite ends of its inner periphery in the axial direction of the sealing unit. Magnetic fluid is held within each gap between its concave periphery and a rotating bushing. In this sealing unit, there is no special means to concentrate the magnetic flux passing through the gap, and accordingly the density of the magnetic flux in the gap is not very high. This type of the sealing unit cannot provide a complete seal to maintain a great pressure difference between two adjoining environments.
In order to increase the effective pressure difference, a plurality of polepieces each having a knife edge, defining a sealing gap, opposite to the rotating bushing may be disposed on opposite sides of the permanent magnet as shown in FIG. 6. In this design, a plurality of sealing barriers of magnetic fluid are separately formed in the axial direction of the bushing. FIG. 7 in the same patent discloses a rotating bushing having a plurality of knife edges to form a plurality of sealing barriers for the same purpose as the sealing device in FIG. 6.
In these prior examples, the magnetic flux is moderately concentrated at the knife edges of the polepieces or the bushing. However, the degree of its concentration is not enough for the sealing devices to maintain a great pressure difference in spite of only a small number of sealing barriers.
Furthermore, in these prior examples, each barrier formed by magnetic fluid is adapted to maintain almost the same pressure difference. However, as each barrier is not strong enough if these prior examples are used in the environment of a high vacuum, it is likely that air between the adjoining barriers in those devices leaks into the vacuum environment. That is, these prior examples can maintain a certain degree of pressure difference as a whole. However, air held between the adjoining barriers is apt to leak into the vacuum environment. Accordingly, a high vacuum environment cannot be created by those prior examples.